Visit the new documentation for updated content. DB4O. Table of contents. 1 Info; 2 The Basic Principle of db4o; 3 Downloads; 4 Current Status. This tutorial was written to get you started with db4o as quickly as possible. Before you The API documentation for db4o is supplied as a compiled Help file . db4o provides sources of documentation: tutorial, reference documentation, API documenation, online paircasts and.
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Using the particular methods of Person would require a downcast on the objects returned by next. However, this mode has limitations. Query Evaluation Mode This property controls when and how much of a query is executed. Running db4o requires only the.
All field objects including collections are saved automatically.
A graph is a network of objects which represent a segment of data. Failing to include these libraries will not generate an error of any kind but will simply cause every native query to be unoptimized.
So you could Descend into an object, and then Execute at a deeper level to return only the matching child objects, like in this example:. When the environment is configured to run with reduced trust, all basic database operations require at least ReflectPermission MemberAccess and ReflectionEmit for the classes and types being persisted.
When accessing a child object, db4o can automatically activate the child object from the database if it is not yet already activated. Regardless of when it is run, query constraints against non-indexed data are always performed on the currently stored object state.
Getting Started With db4o
Optimization is still done inside the database, but instead of text being returned real objects are sent back, ready for use. We said earlier that when calling Store to update an object graph, that db4o will not by default descend into child objects to detect changes to the graph.
QBE is the preferred mechanism for simple query situations because it’s an easier API to work with, but it does require that your domain objects permit any or all of the fields containing data to be set to null, which may violate some of your domain rules.
If a field in the prototype is “null,” then that value matches against any value in the data store; otherwise, the values must match exactly. To close the transaction, use the Commit method:.
NET Framework version 3. Global Update Depth We said earlier that when calling Store to update an object graph, that db4o will not by default descend into child objects to detect changes to the graph. Object data is activated as the users accesses each object in the result set.
Should multiple field values be specified, then all the field values must be met by the object in the database for the candidate object to meet the query criteria; in essence, this means that the fields are “AND”-ed together to form the query predicate.
The second wave of growth was observed in the first decade of the 21st century, when object-oriented databases written completely in an object-oriented language appeared on the market. From the db4o version 7. This article contains content that is written like an advertisement. Clearly the first option above is only viable for the most trivial of databases because it puts an obvious upper-bound on the size of the database that you can practically use.
Ted Neward Published on March 27, Please improve this by adding secondary or tertiary sources. The result set can be influenced by subsequent transactions in the current context or by other clients causing possible concurrency errors.
Depending on the features your application requires, reference and distribute these assemblies when you distribute your application:.
This article relies too much on references to primary sources. With an ActivationDepth of 5, objects will be populated up to 5 levels deep. Once that’s all done, I call get on the db4o database and check to see if the ObjectSet returned contains any objects.
Actually, much of the utility method’s utility become apparent when placed documentattion the stored class itself, as shown in Listing UpdateDepth [depth] ; The default docuumentation 1 means db4o will not descend into child objects when updating stored object instances.
As always, see the db4o documentation for the complete details of the ExtObjectContainer class. A list of object ID matches is generated completely when the query is executed and held in memory Lazy Best for limited resource environments. Use the Visual Studio “Add Reference” command to add the necessary assemblies.
What is apparent from Listing 7, however, is that its usage is arguably not much simpler than the basic QBE version shown already:.
Queries, updates, and identity
This series is a working introduction to db4o, an open source database that leverages today’s object-oriented languages, systems, and mindset. Although this makes for a fairly easy way to issue queries, it also makes for somewhat limited options; for example, what if you needed to retrieve all Person s whose last name started with Gor all Person s of an age greater than 21?
The db4o Java distribution includes several jar files, including a core db4o implementation for each of the JDK 1. As I explained, an object database like db4o simply has more to offer to object-oriented developers, in today’s object-oriented world, than its relational cousins.
Db4o – Wikipedia
Because db4o uses the native object or referential identity, it’s better to open the database or connection when your application begins, do all your db4k, then close the database when your program is shutting down. Setting the ActivationDepth too aggressively can cause poor db4o query performance and high memory usage.